In a radio frequency system, the signal is often modulated by a selected modulation method and the modulated signal is then converted to a higher frequency so as to be transmitted at a designated band. For example, in the North America, the conventional television signal is modulated using VSB modulation and the modulated television signal is up converted to VHF or UHF band for transmission. On the receiver side, the received radio frequency (RF) signal is down converted to a zero-IF, low-IF or IF signal for further processing. The use of down conversion in a receiver system converts the high frequency RF signal down to a lower frequency zero-IF, low-IF or IF signal. As is well known in the field of electronic circuit, a high performance circuit, such as an amplifier or a filter, is harder to implement in a higher frequency than in a lower frequency. The use of down conversion will ease the implementation of receiver circuit. Another great benefit of down conversion is that the converted zero-IF, low-IF and IF signals are more suited for digital processing where the receiver may take advantage of flexibility and programmability offered by digital signal processing. Therefore, up conversion has been widely used in a transmitter and down conversion has been widely used in a receiver.
In recent years, direct-conversion, i.e., zero-IF conversion, and down conversion to low-IF have been preferred architecture for communication receivers because a lower speed digital signal processing may be used to further process the converted signal. In a signal conversion system, a received RF signal is mixed with a mixing signal, typically generated by a local oscillator (LO), to generate a mixed signal. FIG. 1 illustrates a block diagram for a radio frequency receiver 100 having conventional zero-IF signal mixing. The RF input signal 105 is amplified by the variable-gain low-noise amplifier (LNA) 110. The amplified RF signal splits into two paths to feed two mixers 120a and 120b. The mixers receive in-phase 125a and quadrature phase 125b LO signals, where the LO signals are derived from a voltage controlled oscillator (VCO) 150 and the phase shifter 160 generates the needed in-phase 125a and quadrature phase 125b LO signals. The down converted signals are filtered using filters 130a and 130b and subsequently digitized by analog-to-digital converters (ADCs) 140a and 140b. The digital outputs from the ADCs 140a and 140b are then processed using digital processing technology. The LO has to be capable of supplying an oscillating signal with a nominal frequency over a range according to the requirement of system design. For example, the nominal RF frequency is designated as 900 MHz for GSM cellular system and 2.4 GHz for Bluetooth. For television signals, the nominal frequency is designated as VHF (44-92 MHz and 167-230 MHz) and UHF bands (470-860 MHz). In a quadrature mixing system, there is a need to generate LO signals for both I and Q channels which have a 90° phase difference. For a differential signal system, a differential VCO is used which can only provide differential output and a divide-by-2 circuit is often used to generate I and Q signals with 90° phase difference. Therefore, the VCO has to generate a frequency which is two times the desired LO frequency of VCO frequency. For example, VCO has to work at 1800 MHz for the GSM system and 4.8 GHz for the Bluetooth system. For broadcast television system, the VCO has to work up to 1720 MHz. The higher frequency implies higher power consumption and higher design complexity.
FIG. 2 illustrates a circuit of double balanced mixer for a conventional receiver system 200. The mixer includes four MOS transistors Q1 202, Q2 204, Q3 206 and Q4 208, where the output signals VOP and VON are connected to the drains of Q1 and Q2 respectively. The MOS transistors Q1-Q4 are controlled by the LO signal pair LOP and LON, where the LO signal pair is applied to the gates of MOS transistors Q1-Q4. The input signal pair VIP and VIN is applied to the gates of MOS transistor Q5 212 and Q6 214. The mixer circuit also includes load resistors R1222 and R2 224 and resistors R3 226 and R4 228. A power AVDD is connected to the MOS transistors Q1-Q4 through the load resistor pair R1 and R2. The MOS transistors Q1-Q4 are utilized as switches to be turned ON and OFF by the LO signal. The effect of switching ON and OFF the input signal by the LO signal is equivalent to multiplying the input signal with harmonics of the LO signal.
Besides the required frequency of the LO, the tuning range of the LO also plays an important role in the LO design. For example, LO for TV receivers with tuning range from 44 MHz to 860 MHz is hard to design due to the extremely wide tuning range. In order to overcome such challenge, usually separate LO circuits are used for VHF and UHF bands. Even so, the design for such wide tuning system is still a challenge. For example, the UHF band television signal will require a tuning range from about 450 MHz to 900 MHz for a zero-IF system, where the tuning range is approximated in order to simplify the discussion. The tuning range may also refer to a ratio of the difference between the highest frequency and the lowest frequency to the center frequency. For the above example, the difference is 450 MHz and the center frequency is 675 MHz, which leads to a 66.6% tuning range. For the UHF band, this tuning range presents a challenge to hardware implementation using on-chip inductors due to the parasitic capacitors and process variations. For an LO frequency plan having a divide-by-2 divider for the UHF band television signal is shown in Table 1 for zero-IF signal mixing.
TABLE 1LO frequency plan for divide-by-2 structure, where the unit forfrequency is in MHzRF: 450-900VCOLO = VCO/2Min 900450Max1800900Tuning range66.7%
In order to reduce the wide tuning range, it has been described in the literature of an LO system using higher VCO frequency followed by divide-by-4 and divide-by-6 dividers to generate differential LO signals for 1 and Q channels. This method reduces the tuning range of the VCO from 66.7% to 40%. As shown in Table 2, the higher frequency range, 600-900 MHz, is generated by the divide-by-4 divider while the lower frequency range, 450-600 MHz, is generated by the divide-by-6 divider, which can cover as low as 400 MHz. Therefore, the tuning range for the VCO is reduced to 40%. However, the new working frequency range, 2400-3600 MHz, becomes much higher than before and the higher VCO frequency implies higher power consumption.
TABLE 2The LO frequency plan for divde-by-4 and divide-by-6 structure,where the unit for frequency is in MHzRF: 450-900VCOLO = VCO/4LO = VCO/6Min2400600400Max3600900600Tuning range40%
Accordingly, there is a need to develop system and method for wide-band signal mixing that has reduced tuning range without increasing the power consumption. For example, if the third-order harmonic of the LO signal can be used to generate a third-order harmonic mixed signal based on the frequency plan of Table 2, the LO signal will be in the range from 200 to 300 MHz for the divide-by-4 divider and 133.3 to 200 MHz for the divide-by-6 divider. The required VCO frequency range will become 800 to 1200 MHz, which is much lower than the frequency range required by a conventional approach.